Willow bark has been used as a treatment for pain and fever in China since 500 BC. In Europe, it was primarily used for altogether different purposes, such as stopping vomiting, removing warts, and suppressing sexual desire. However, in 1828, European chemists made a discovery that would bring together some of these different uses. They extracted the substance salicin from white willow, which was soon purified to salicylic acid. Salicylic acid is an effective treatment for pain and fever, but it is also sufficiently irritating to do a good job of burning off warts.

Chemists later modified salicylic acid (this time from the herb meadowsweet) to create acetylsalicylic acid, or aspirin.

Aspirin and related anti-inflammatory drugs are notorious for irritating or damaging the stomach. However, when taken in typical doses, willow does not appear to produce this side effect to the same extent.
1,2
This may be partly due to the fact that most of the salicylic acid provided by white willow comes from salicin and other chemicals that are only converted to salicylic acid after absorption into the body.
3
Other evidence suggests that standard doses of willow bark are the equivalent of one baby aspirin daily rather than a full dose.
4

This latter finding raises an interesting question: If willow provides only a small amount of salicylic acid, how can it work? The most likely answer seems to be that other constituents besides salicin play a role. Another possibility may be that the studies finding benefit were flawed, and that it actually does not work.

What Is the Scientific Evidence for Willow?

In a 4-week,
double-blind, placebo-controlled study
of 210 individuals with
back pain
, two doses of willow bark extract were compared against placebo.
4
The higher-dose group received extract supplying 240 mg of salicin daily; in this group, 39% were pain-free for at least 5 days of the last week of the study. In the lower-dose group (120 mg salicin daily), 21% became pain free. In contrast, only 6% of those given placebo became pain-free. Stomach distress did not occur in this study. The only significant side effect seen was an allergic reaction in one participant given willow.

Benefits were also seen in a double-blind, placebo-controlled trial of 78 individuals with
osteoarthritis
of the knee or hip.
6

However, two subsequent double-blind, placebo-controlled studies performed by a single research group failed to find white willow more effective than placebo.
10
One enrolled 127 people with osteoarthritis (OA) of the hip or knee, the other 26 outpatients with active
rheumatoid arthritis
(RA). In the OA trial, participants received either willow bark extract (240 mg of salicin per day), the standard drug diclofenac (100 mg per day), or placebo. In the RA trial, participants received either willow bark extract at the same dose or placebo. While diclofenac proved significantly more effective than placebo in the OA trial, willow bark did not. It also failed to prove more effective than placebo in the RA study.

The most likely interpretation of these conflicting findings is that willow provides at best no more than a modest level of pain relief.

Dosage

Safety Issues

Evidence suggests that willow, taken at standard doses, is the equivalent of 50 mg of aspirin, a very small dose.
4
Willow doesn't impair blood coagulation to the same extent as aspirin,
8
and also doesn't appear to significantly irritate the stomach.
9
Nonetheless, it seems reasonable to suppose that, if it is used over the long term or in high doses, willow could still cause the side effects associated with aspirin. All the risks of aspirin therapy potentially apply.

For this reason, white willow should not be given to children, due to the risk of Reye's syndrome. It should also not be used by people with aspirin allergies, bleeding disorders, or kidney disease. In addition, it may interact adversely with "blood thinners," other anti-inflammatory drugs, methotrexate, metoclopramide, phenytoin, probenecid, spironolactone, and valproate.

Safety in pregnant or nursing women, or those with severe liver or kidney disease, has not been established.

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.